The present invention relates to a process for the post-treatment of developed relief printing forms for use in flexographic printing, which have been reexposed in a customary manner.
Photosensitive flexographic printing plates, in general, comprise a multilayer material. In most cases, they are formed of a dimensionally stable support; an adhesive layer; a photopolymerizable or photo-crosslinkable elastomeric layer, usually 0.5 to 7 mm thick, containing an elastomeric binder, polymerizable monomers, a photoinitiator and, optionally, additive substances; a non-tacky covering layer (polymer overcoat); and a detachable protective film.
The support may be made of a metal or, preferably, of a dimensionally stable, transparent plastic film. It serves to reinforce and maintain the dimensional stability of the elastomeric layer, which is optionally up to several millimeters thick, in the printing process. To obtain firm anchoring of the light-hardenable layer to the support an adhesive layer is required, which comprises, for example, a polyurethane or polychloroprene-based one-component or two-component adhesive, as described in DE-A No. 31 00 175, DE-A No. 31 07 741, DE-A No. 24 44 118 and DE-A No. 31 37 416, a polymerizable layer (U.S. Pat. No. 3,036,913) or a mixture of polyvinyl chloride and polyvinyl acetate (DE-A No. 33 19 676). EP-A No. 64 564 discloses mixtures of specific polyesters and polyamides, polyurethanes or polyolefins, which are used for the same purpose. Adhesive layers comprising a chlorosulfonated polyethylene are disclosed in German Patent Application, file No. P 37 40 432.6.
Upon imagewise exposure of photosensitive flexographic printing plates of this type, the image areas of the photopolymer layer are photopolymerized or photocrosslinked. After washing out the non-hardened layer areas by means of a suitable solvent a rubber-elastic relief printing form is obtained, which, as a rule, can be further hardened by reexposure to actinic light.
At the layer surface of the photopolymer layer photopolymerization proceeds incompletely, which is presumably caused by the increased concentration of oxygen present in this area. Incomplete polymerization is discernible by a tacky plate surface.
A tacky surface is inconvenient when the plates are used as printing plates. In the printing process, for example, the fluff of paper adheres to the tacky plates and, as a result, the printing quality is reduced. On the other hand, tacky plates arranged in a stack can be separated from one another only with difficulty.
Various methods can be employed to eliminate inconvenient surface tackiness.
Post-treatment with an aqueous solution containing free halogen for substances that release halogen is customary in practice. The plate can, for example, be immersed into an aqueous solution of NaOCl and HCl or treated with a bromine solution (DE-C No. 28 23 300), with a bromine solution containing a wetting agent (DE-A No. 35 21 955) or with an iodine solution (DE-A No. 30 71 141). A double treatment, i.e. first in a bromine or iodine bath and then in a NaOCl/HCl bath has also been recommended (EP-A No. 0 096 835=U.S. Pat. No. 4,451,553).
The solutions used in these processes are, however, aggressive and have a corrosive effect.
Apart from this, the printing form can also be treated with a solution of carbonyl compounds forming free radicals under the action of UV light, followed by irradiation with short-wave UV light (U.S. Pat. No. 4,202,696). This process, however, has the disadvantage that it uses UV light (200-300 nm) which is injurious to health and which requires special lamps for generation. In addition, the exposure times are relatively long (up to 15 minutes).
According to another method the tacky printing form is exposed to UV light, while being immersed in an aqueous bath containing photopolymerizable monomers and photoinitiators (WO 86/02177).
It is a disadvantage of this method that an expensive special apparatus is necessary and that the solutions or dispersions used are not stable.
Moreover, the printing form can be coated with a solution containing non-tacky binders, photopolymerizable monomers and photoinitiators and subsequently exposed to UV light (DE-A No. 37 04 067). These coatings are, however, relatively hard and reduce the flexibility of the printing form.
The most widely used post-treatment in a bromine-containing bath not only has the above-described disadvantages but also intensifies the printing form's tendency to develop cracks when in contact with ozone-containing air. Crack formation can render the printing form useless. The number and severity of the cracks are dependent on the composition of the elastomeric photopolymer layer. By using antiozonants, plasticizers and elastomers that do not contain olefinic double bonds the tendency to crack formation can be reduced. It is, however, difficult to find suitable compatible and non-toxic compositions that, upon photopolymerization, yield relief printing forms having good mechanical properties and, at the same time, a low sensitivity to ozone.
In nearly all commercial flexographic printing plates comprising photopolymers resistance to ozone needs improving, even in those cases in which the photopolymer layer contains a high proportion of antiozonants. Various post-treating methods have therefore been proposed, which are intended to prevent crack formation during storage of printing forms that have already been rendered non-tacky.
A process of this kind concerns the treatment of printing forms with solutions of nickel or zinc dialkyldithiocarbamates in an organic solvent (EP-A No. 0 137 331). The solutions used have, however, the disadvantage of being physiologically unsafe.
According to other methods the relief printing form is coated with an alcoholic polyamide solution (DE-A No. 34 15 044) or treated with liquid polyglycol ethers (DE-A No. 35 12 632). These methods may effect protection of the printing form before or even after the printing operation, but during the printing operation the substances protecting the printing form from ozone are washed out by the solvents of the printing inks.